The remarkable surface homogeneity of the Dawn mission target (1) Ceres

Dwarf-planet (1) Ceres is one of the two targets, along with (4) Vesta, that will be studied by the NASA Dawn spacecraft via imaging, visible and near-infrared spectroscopy, and gamma-ray and neutron spectroscopy. While Ceres' visible and near-infrared disk-integrated spectra have been well characterized, little has been done about quantifying spectral variations over the surface. Any spectral variation would give us insights on the geographical variation of the composition and/or the surface age. The only work so far was that of Rivkin & Volquardsen (2010, Icarus 206, 327) who reported rotationally-resolved spectroscopic (disk-integrated) observations in the 2.2-4.0 {\mu}m range; their observations showed evidence for a relatively uniform surface. Here, we report disk-resolved observations of Ceres with SINFONI (ESO VLT) in the 1.17-1.32 {\mu}m and 1.45-2.35 {\mu}m wavelength ranges. The observations were made under excellent seeing conditions (0.6"), allowing us to reach a spatial resolution of ~75 km on Ceres' surface. We do not find any spectral variation above a 3% level, suggesting a homogeneous surface at our spatial resolution. Slight variations (about 2%) of the spectral slope are detected, geographically correlated with the albedo markings reported from the analysis of the HST and Keck disk-resolved images of Ceres (Li et al., 2006, Icarus 182, 143; Carry et al., 2008, A&A 478, 235). Given the lack of constraints on the surface composition of Ceres, however, we cannot assert the causes of these variations.Comment: 8 pages, 5 figures, 2 tables, accepted for publication in Icaru

Compression and Sintering of Powder Mixtures: Experiments and Modelling

International audienceIn numerous industrial fields, materials are manufactured from powders. The most classical process consists in pouring the powder into a die and pressing in between two punches. The resulting component can be used in this state, as in the pharmaceutical industry. But, most often, for example in powder metallurgy or ceramic processing, it is submitted to a thermal treatment, called sintering, during which the particles are welded together. These two stages, compaction and sintering, have been extensively investigated in the last decades. The main physical mechanisms arising during both stages have been identified and more or less sophisticated models have been developed. However most of these studies have concerned single-component powders whereas industrial processes more frequently use mixtures of powders with different physical and mechanical. The interest of using mixtures instead of single-component powders may be to facilitate the compaction or the sintering or to create alloys or composite materials with outstanding features. Understanding and modelling the behavior of powder mixtures require taking into account the mechanical and chemical interactions between both phases. Concerning mechanical problems, in most cases, a simple law of mixture is not appropriate. Classical models of multiphase materials hardly give better results. Accounting for the granular nature of the material is thus absolutely necessary. Relevant issues are interparticle contacts, particle rearrangement, agglomeration, phase percolation. The compressibility of bimodal mixtures, for example, is mainly related with the ratio of the size of particles of one phase to the size of particles of the other phase. Chemical phenomena involve more or less complex phase transformations and chemical reactions. Such transformations or reactions usually occur during sintering but they can also be generated when the material is still in the powder state, for example during milling. Most research studies in the field of powder processing at the Federation concern powder mixtures. Three examples are presented in the following, illustrating recent research studies on the behavior of powder mixtures during compression or sintering

Phosphine-Catalyzed Synthesis of 3,3-Spirocyclopenteneoxindoles from γ‑Substituted Allenoates: Systematic Studies and Targeted Applications

The phosphine-promoted [3 + 2] cyclizations between γ-substituted allenoates and arylideneoxindoles have been applied to the stereoselective synthesis of spiro­(cyclopentene)­oxindoles with trisubstituted cyclopentene units. It has been demonstrated that PPh₃ operates a very efficient control of the relative stereochemistry of the three stereogenic centers of the final spiranic products. Focused experiments have been carried out then so as to access carbocyclic analogues of an important series of anticancer agents inhibiting MDM2-p53 interactions

Phosphine-Catalyzed Synthesis of 3,3-Spirocyclopenteneoxindoles from γ‑Substituted Allenoates: Systematic Studies and Targeted Applications

The phosphine-promoted [3 + 2] cyclizations between γ-substituted allenoates and arylideneoxindoles have been applied to the stereoselective synthesis of spiro­(cyclopentene)­oxindoles with trisubstituted cyclopentene units. It has been demonstrated that PPh₃ operates a very efficient control of the relative stereochemistry of the three stereogenic centers of the final spiranic products. Focused experiments have been carried out then so as to access carbocyclic analogues of an important series of anticancer agents inhibiting MDM2-p53 interactions

Gaia contribution to the dynamics of Solar System Objects

International audienceThe ESA astrometric mission Gaia has been launched in December 2013. It is currently in its commissioning phase, with the first scientific data expected to be downloaded in June 2014. Gaia has the capability to observe, in addition to about one billion of stars, a large number of solar system objects (SSO) [1]. The satellite and telescope will continuously scan the sky during 5 years, providing high precision astrometry and photometry for about 300,000 asteroids (and several tens of planetary satellites and comets), as well as modest imaging for a fraction of them. The nominal limiting magnitude is expected to be

Gaia contribution to the dynamics of Solar System Objects

International audienceThe ESA astrometric mission Gaia has been launched in December 2013. It is currently in its commissioning phase, with the first scientific data expected to be downloaded in June 2014. Gaia has the capability to observe, in addition to about one billion of stars, a large number of solar system objects (SSO) [1]. The satellite and telescope will continuously scan the sky during 5 years, providing high precision astrometry and photometry for about 300,000 asteroids (and several tens of planetary satellites and comets), as well as modest imaging for a fraction of them. The nominal limiting magnitude is expected to be

2015 KM18

International audienceAvailable from the Minor Planet Center

2015 KM18

International audienceAvailable from the Minor Planet Center